Aspheric optical lenses are conventionally polished with continuous precession by 5-axis bonnet polishing system. In this work, in order to achieve that aspheric lenses be processed by four-axis simultaneous NC (numerical control) machining system, one kind of control algorithm was proposed by controlling the error of precession angle. Firstly, on the basis of the analysis of the structure of bonnet tool and processing technology of aspheric optics, ranges of important parameters in machining process such as the angle between spin axis of bonnet tool and negative vertical direction and the precession angle were fixed, to make the minimum errors of precession angle as small as possible; Secondly, the control algorithm was introduced, aimed to reduce the existing errors of precession angle in polishing process by making micro migration of machining points. Then, simulation experiments of polishing non-axisymmetric aspheric lenses were carried out to validate the suggested control algorithm, the results revealed that the maximum error of precession angle was decreased to about $0.01^{circ}$, which is available in real processing.